1. Field of the Invention
The invention concerns the technical field of so-called “dry” reinforcement materials, adapted to the creation of composite parts in association with a thermosetting or thermoplastic resin which will be diffused within said materials to create the final composite part. Specifically, the invention relates to a new intermediate material based on unidirectional sheets of carbon fibre, combining satisfactory mechanical strength and electrical conductivity properties, adapted to the design of composite parts by subsequent injection or infusion of a thermosetting or thermoplastic resin, or of a mixture of such resins, and relates as well to a process for the fabrication of composite parts from such a material, and to the composite parts thus obtained.
2. Description of Related Art
The fabrication of composite parts or products, that is to say consisting of, first of all, one or more fibrous reinforcements or sheets, and second of all, a matrix primarily of the thermosetting (“resin”) type, which may include thermoplastics, can for example be performed with a process called “direct” or “LCM” (“Liquid Composite Moulding” in English). A direct process is defined by the fact that one or several fibre reinforcements are prepared in a “dry” state (that is, without the final matrix), the resin or the matrix being prepared separately, for example by injection into the mould containing the reinforcement fibres (“RTM” process, Resin Transfer Moulding), by infusion through the thickness of the reinforcement fibres (“LRI” process, “Liquid Resin Infusion” in English or “RFI” process, “Resin Film Infusion”), or else by manual coating/impregnation with a roller or brush on each of the unit layers of fibrous reinforcement, applied successively on the mould.
For the RTM, LRI or RFI processes, it is generally first necessary to build a fibrous preform in the shape of the desired finished product, then to impregnate this preform with a resin. The resin is injected or infused by differential pressure at temperature, then once all the amount of necessary resin is contained in the preform, the assembly is brought to a higher temperature to complete the polymerization/crosslinking cycle and thus harden it.
Composite parts used in the automobile, aviation or naval industry are particularly subject to very strict requirements, notably in terms of their mechanical properties. To conserve fuel, the aviation industry has replaced many metallic materials with composite materials that are lighter. In addition, many hydraulic flight controls are replaced by electronic controls also in the interest of weight reduction.
The resin which is subsequently associated, in particular by injection or infusion, to the unidirectional reinforcement sheets during the creation of the part can be a thermosetting resin, an epoxy for example. To allow proper flow through a preform consisting of a stack of different layers of carbon fibres, the resin is most often very fluid, for example with a viscosity on the order of 50 to 200 mPa·s. at the infusion/injection temperature. The major inconvenience of this type of resin is its fragility after polymerization/crosslinking, which results in poor impact resistance of the fabricated composite parts.
In order to solve this problem, the documents of previous art proposed the association of the unidirectional layers of carbon fibres to intermediate layers based on resin, and notably to a web of thermoplastic fibres. Solutions such as these are notably described in patent applications or patents EP 1125728, U.S. Pat. No. 6,828,016, WO 00/58083, WO 2007/015706, WO 2006/121961 and U.S. Pat. No. 6,503,856. The addition of this intermediate layer of resin, such as a web, makes it possible to improve mechanical properties in the compression after impact (CAI) test commonly used to characterize the impact resistance of structures.
In the earlier patent applications WO 2010/046609 and WO 2010/061114, the applicant has also proposed particular intermediate materials comprising a sheet of unidirectional fibres, particularly carbon, associated by adhesion on each of its faces to a non-woven of thermoplastic fibres (also called non-woven), as well as their preparation process.
Such composite materials consist of layers of carbon and layers of thermosetting or thermoplastic material. The carbon fibre is electrically conductive, unlike the thermosetting or thermoplastic materials. The stack of these two materials is thus a stack of conductive materials and insulating materials. The transverse electrical conductivity is near-zero due to the presence of resin layers.
However, in order to dissipate the energy of lightning moving over the fuselage or the wings, and also to assure the function of return current, the transverse electrical conductivity of composite parts used in aviation must be high. Because fuel reserves are located in the wings of planes, it is essential to successfully dissipate the electrical energy and therefore to achieve good conductivity along the axis orthogonal to the surface of the part, called the z-axis. In aircraft structures, electrical conductivity has been provided until now by the material itself, which was mostly based on aluminium. Because the new aircraft models integrate more and more composite materials, mainly based on carbon, it has become essential to provide additional conductivity to assure the functions of return current and resistance to lightning. This conductivity is achieved currently on composite parts based on carbon fibres by the local use of metallic ribbons or rovings that bind the parts to each other. Such a solution greatly increases the weight and cost of the composite solution, and is therefore not satisfactory.
In order to provide materials having a good electrical conductivity, patent application U.S. 2003/0008125 provides for associating a sheet of unidirectional carbon fibres with a carbon non-woven and assuring their bonding with a thermoplastic layer. The thermoplastic layer is used to impregnate the carbon layers. The disclosed material, in particular because of the use of a non-porous thermoplastic layer and the lack of precision regarding the amount of thermoplastic resin present is not suitable for use in direct processes such as described above.
Patent application WO 99/21697 also provides for associating unidirectional carbon sheets with carbon non-wovens, but only in order to provide materials suitable for direct processes. For this, the association between the various layers is assured by a grid made of a polymerizable material, that is to say, thermosetting, so as to limit the amount of polymerizable material present to 6 to 12% by weight of the total weight of the material.